1. Technical Field
The technology described herein is generally related to the field of power distribution and more particularly to delaying activation of a current limiter in order to allow temporary surge currents.
2. Description of Related Art
Electronic appliances often require protection against power surges which could result in faulty operation or damage. Particularly for sensitive electronics such as computers and portable telecommunications devices using semiconductor components, when electrical power is applied some mechanism for limiting the power—particularly the electrical current being drawn—is advantageous. Such mechanisms can both protect the instrument's power supply from being drained of all energy and against the possibility of a start-up fault, damage to the unit, or even a fire in the event of a severe load or electrical fault associated with the power supply. It is known in the art to use a variety of fault protection methods and devices such as fuses, resettable fuses and circuit breakers, polymeric positive temperature coefficient devices, current limiter circuitry, and the like.
However, not all legitimate loads behave in the same manner. Some loads such as incandescent lights and motors are known to draw a substantially larger electrical current upon start-up—commonly referred to as a “surge current”—than is required for steady-state operation. The known manner fault protection methods and devices may restrict or cut-off a current to the load prematurely during such a surge, causing the load to stall or to fail altogether. Certain load appliances like disk drives, charge-coupled device (CCD) cameras, scanners and the like may actually require large, extended transient electrical current during a start-up routine.
FIG. 1 (Prior Art) shows a typical CURRENT LIMITER 100 attached to a LOAD 101. Assume for this explanation that the LOAD 101 is an appliance connected to a power supply, VDD, using a metal oxide semiconductor field effect transistor (MOSFET) ON-OFF power distribution switch 103. A known manner CURRENT SENSE circuit 105 senses the current through the power distribution switch 103. The CURRENT SENSE circuit 105 provides a signal indicative of real-time current through the switch 103 to a CURRENT LIMIT AMPLIFIER 107. The current sense signal to the positive side input of the CURRENT LIMIT AMPLIFIER 107 is compared therein to a REFERENCE signal generator 109 output signal defining a predetermined threshold for recognizing an over-current condition at the switch 103. If an over-current state is detected, the CURRENT LIMIT AMPLIFIER 107 provides an output signal to a GATE DRIVE CIRCUIT 111. This results in a gate drive signal output to the power distribution switch 103 being decreased, increasing the power distribution switch ON-resistance, thereby limiting the current to the LOAD 101.
With the typical CURRENT LIMITER 100 there is a delay—also known in the art as the “current limit delay time”—between when the over-current state is detected and when the power distribution switch 103 begins limiting the current to the LOAD 101. The current limit delay time is dependent on the response time of the above-described current limiter circuitry control loop components. The current limit delay time, “CLDT,” may be expressed as:CLDT=CLRT,where CLRT is the reaction time of the circuitry making up the CURRENT LIMITER 100. In typical applications, such delay times are in the range of about tens-to-hundreds of microseconds. However, for some load applications, start-up currents are needed that are actually larger than the desired current limit threshold defined by the REFERENCE 109 and have start-up times in the tens-to-hundreds of milliseconds range. The relatively faster current limit delay times of the typical CURRENT LIMITER 100 may not allow such applications to start correctly. To avoid this problem, some existing current limiters set their current limit thresholds high enough to allow a surge current without limiting. However, with the current limit threshold set that high, the current limiter may fail to detect a legitimate, steady state, over-current condition.
In other words, existing CURRENT LIMITERS 100 either act too fast or have a current limit threshold set so high that a legitimate over-current condition may not be detected.
The present invention addresses these and other related problems.